1. Field of the Invention
This invention relates to a recording and/or reproducing apparatus such as a video tape recorder (VTR) arranged to permit selective mounting of tape cassettes of different types having different distance between tape winding reels.
2. Description of the Related Art
Some VTRs which have heretofore been proposed and practicalized are arranged to permit selective mounting of tape cassettes of different types having different distances between reels for different purposes, namely, tape cassettes of two types. One is a large-sized cassette arranged to permit recording and/or reproduction over a long period of time. The other is a small-sized cassette arranged by attaching importance to portability. Generally, the VTR of this kind is arranged to permit the selective mounting of the tape cassette of any of the different types by moving and adjusting a pair of reel mounts to a pair of reels disposed within the cassette. For this arrangement, some tape tension control mechanisms also have been proposed.
According to a known tape tension control method which has been developed for the tape tension control mechanism, the pair of reel mounts are driven directly by different motors and the tape tension is controlled by individually controlling the two reel motors. This method has a good controllability for the tape tension. However, this method results in an expensive system and has not been used for ordinary home (or household) VTRs. For home VTRs, a method of mechanically controlling the brake torque of a tape-supply-side reel is employed in general.
A mechanical tape tension control mechanism which is popularly in use is described with reference to FIG. 1 as follows. A supply-side reel mount 204 is provided with a brake drum 244 which is disposed on the circumference of the supply-side reel mount 204. A magnetic tape 203 is pulled out from a supply reel which is not shown but is mounted on the supply-side reel mount 204. A tension arm 218 is swingable on a support shaft 217. A tension detecting guide 245 is provided at the fore end of the tension arm 218. A tension spring 216 urges the tension arm 218 to swing counterclockwise. One end of the tension spring 216 is secured to a support part 215 and the other end to a support part 250 which is provided on the tension arm 218. A brake band 213 is wrapped around the brake drum 244 and is arranged to generate a brake torque. One end of the brake band 213 is secured to a support part 211 and the other end to a support part 214 which is provided on the tension arm 218. Reference numerals 228 and 229 denote tape guides.
The operation principle of the tape tension control mechanism which is arranged as described above is as follows. The tape 203 which is pulled out from the supply reel mounted on the supply-side reel mount 204 reaches the tension detecting guide 245 via the tape guide 228 and is then led further to the tape guide 229 to form a predetermined tape path. After the tape path is thus formed, the magnetic tape 203 is taken up and wound around a takeup reel which is not shown. The tension arm 218 receives a clockwise moment from a tape tension applied to the tension detecting guide 245 and a counterclockwise moment from the spring 216. The tension arm 218 receives also a clockwise moment from the band tension of the brake band 213. The tension arm 218 takes repose in a position where the three moments balance. Meanwhile, the brake drum 244 on the supply-side reel mount 204 is tightened by the tension of the brake band 213 to generate a brake torque, which is applied to the supply reel.
When the tension of the tape 203 increases under the above-stated condition, the clockwise moment of the tension arm 218 increases. Then, the tensile force of the spring 216 increases and the band tension of the brake band 213 decreases in such a manner that the moments associated with the support shaft 217 serving as the rotational center of the tension arm 216 balance. When the band tension decreases, the brake torque generated at the brake drum 244 decreases to decrease the tape tension. With the tape tension decreased, the control is carried on to increase the tape tension by reversely performing the above-stated actions.
The balance of moments around the tension arm gives the following relation: EQU MS=FB LB+T LT (1)
wherein
MS: the moment of rotation caused by the spring PA1 FB: the band tension PA1 LB: an effective distance relative to the moment of rotation caused by the band tension about the center of swing of the tension arm PA1 T: the tape tension PA1 LT: an effective distance relative to the moment of rotation caused by the tape tension about the center of swing of the tension arm PA1 R: the radius of a tape coil on the supply reel PA1 RD: an effective radius relative to a brake torque caused by the band tension PA1 (i) Of the design parameters MS, LB, RD and LT which are necessary for control over the tape tension, the parameters RD and LT are limited by the dimensions of the small-sized tape cassette. PA1 (ii) The tape coil radius R varies at a greater rate on the large-sized tape cassette than on the small-sized tape cassette. Therefore, in order to minimize the rate of variations of the tape tension T resulting from the variations of the tape coil radius, it is necessary to set the parameters RD and LT at large values and the parameter LB at a small value.
The balance of forces around the reel on the other hand give the following relation: EQU T R=RD FB (2)
wherein
Elimination of "FB" from the formula above gives the following formula: EQU T=MS/(((LB.multidot.R)/RD))+LT) (3)
Therefore, the tape tension T is controlled to obtain the value as mentioned above.
As apparent also from the formula (3), when the tape coil radius R on the reel changes, the tape tension T comes to change accordingly. Therefore, other design parameters MS, LB, RD and LT are set in such a way as to minimize the rate of the change.
However, when the conventional mechanical tape tension control mechanism which is generally arranged in the manner described above is applied to the VTR of the kind arranged to permit selective mounting of a large-sized cassette or a small-sized cassette, it brings about the following problems.